1. Field of the Invention
The present invention relates to an image reading apparatus for performing an image reading routine with correction of captured image based on a background color of a source document, and more particularly to such an image reading apparatus that is capable of performing the image reading routine in a reduced length of time.
2. Discussion of Related Art
There is known an image reading apparatus arranged to capture an original image such as character, picture and photograph that is carried on a source document, and to convert the captured image into a digital image data (pixel-related data). Further, as disclosed in JP-H11-39463A, there is proposed a technique of performing a so-called “background-color-basis correction” in such type of image reading apparatus, for avoiding the image data from being influenced by a background color of the source document, particularly, where the background color is non-white color rather than white color.
FIG. 8 schematically shows an original image of a source document having a non-white background color, and a corrected image obtained by capturing the original image and eliminating the non-white background color from the captured original image. For example, in case of conversion to 16-bit digital data from brightness of each pixel of an image captured from a source document whose background color is white, brightness of the brightest pixel (i.e., brightness of white color) is represented by “FFFF” as the maximum value of 16-bit data while brightness of the darkest pixel (i.e., brightness of black color) is represented by “0000” as the minimum value of 16-bit data. On the other hand, in case of conversion to 16-bit digital data from brightness of each pixel of an image read from a source document whose background color is not white, the above-described “background-color-basis correction” is performed such that brightness of the non-white background color is represented by “FFFF” as the maximum value of 16-bit data.
Specifically, the source document is first subjected to pre-scanning for obtaining brightness of the background color of the source document so as to determine a background-color-basis correcting value based on the obtained brightness of the background color. Then, the same source document is subjected to main scanning for obtaining image data, and the image data is corrected by using the determined background-color-basis correcting value. Thus, it is possible to obtain the image data that is not substantially affected by influence of the background color.